The Wolffian duct (WD) is the progenitor for the male internal reproductive tract, which undergoes morphological differentiation from a simple tube-like structure into the epididymis, efferent ductules, and vas deferens. Initial formation of the WD is independent of the sex of the gonad or hormones produced in the gonads; however, later development of the WD requires Leydig cell-derived androgens, whose mechanisms of action remain unknown. It is well proven in human clinical cases and mouse genetic studies that defects in development of the WD have catastrophic consequences leading to intersex and reproductive malignancy. In order to better comprehend the causes of these congenital disorders and the secondary health effects, it is necessary to understand the general molecular mechanisms involved in the development, maintenance and differentiation of the WD. To understand the molecular mechanisms for formation and maintenance of the WD and 2) investigate how the WD differentiates into specialized regions of the male reproductive tract. This proposal will use several genetic mouse models (conditional Shh knockout, Testicular feminization, and activin A knockout) analyzed by methodologies including in situ hybridization, gross and histopathological analysis, and immunocytochemistry. The data obtained from this research is critical in identifying the normal course of sex differentiation, which is fundamentally important to understand how infertility, reproductive neoplasia, and intersexualilty arise. [unreadable] [unreadable] [unreadable]